Ceramic arc plates having minimum density variations



Nov. 15, 1966 A. E. STRINGFELLOW 3,286,065

CERAMIC ARC PLATES HAVING MINIMUM DENSITY VARIATIONS Filed March 16, L966 2 Sheets-Sheet 1 I N VEN TOR. 414 EN 5 576/671710 NOV. 15, 1966 A STRINGFELLQW 3,286,065

CERAMIC ARC PLATES HAVING MINIMUM DENSITY VARIATIONS Filed March 16, was 2 Sheets-Sheet 2 Z/a. I 35 .I 37 I 40 I I I I H H INVENTOR.

. if JJzZ/V 5 Jaw mama E5. 5 BY United States Patent 3,286,065 CERAMIC ARC PLATES HAVING MINIMUM DENSITY VARIATIONS Allen E. Stringfellow, Haddonfield, N.J., assignor to I-T-E Circuit Breaker Company, Philadelphia, Pa., a corporation of Pennsylvania Filed Mar. 16, 1966, Ser. No. 534,699 6 Claims. (Cl. 200-144) This application is a continuation-in-part of United States patent application Serial No. 308,723 filed September 13, 1963, now abandoned, in the name of Allen E. Stringfellow, and assigned to the assignee of the instant invention.

The present invention relates to ceramic arc plates, arc plate assemblies comprising interleaved stacks of such plates and to the method of manufacture of such articles.

As is well known in the art, electrical power circuit interrupting equipment must have are extinguishing means in order to cool off and deionize the are which is formed between the movable interrupting contacts. For relatively low voltage applications, the arc extinguishing means may comprise a plurality of metallic members which, by virtue of their high heat conductivity, cool the arc and prevent restriking thereof. However, for high voltage circuit interrupting equipment, it is necessary to use insulating plates which are spaced so as to provide a tortuous path for the arc and thereby more rapidly extinguish the same.

Such high voltage insulating arc plates are made of ceramic materials having high dielectric strength and resistance to thermal shock. Such materials include zircon, the sillimanite refractories and glass-bonded mica. It is particularly desirable to utilize for the arc plate material a ceramic consisting essentially of from about to 13% magnesium oxide, from about 37% to 57% alumina, and from about 38% to 50% silica, which ceramic contains more than about 50% cordierite crystals. Such material is subject of my prior United States Patent No. 2,864,919. It will be understood that any of the foregoing, or other known ceramics, may be employed to form are plates in accordance with the present invention.

Various procedures have been devised to mold ceramic ware into arc plates suitable for use in arc chutes. In many such procedures the ceramic material is molded into flat arc plates, the resulting plates are rigidly stacked in an arc chute, and the separate spacing materials between the plates are thereafter integrated. Such procedures are costly due to the time required for stacking the several arc plates and spacing materials; more-over, the cumulative arc plate and spacing material tolerances may present significant problems in final assembly.

It is also known to form depressions in flat arc plates or to initially form the plates having channel-shaped cross sections in order to provide oppositely disposed plate faces which, upon direct stacking of a number of plates in an arc plate assembly, are spaced sufficiently to provide a tortuous path for extinguishing the arc of a circuit interrupter. However, the differential densities produced in such plates during manufacture thereof often lead to crack formation, either during the molding operation or when the plate is subsequently subjected to mechanical stress. Moreover, when the relatively brittle ceramic arc plates are rigidly suported in an arc chute shell they are frequently fractured by the mechanical forces accompanying circuit interruption.

It has also been suggested to assemble arc plates, whether of the fiat or channel-shaped types, into an arc plate assembly, the spacing between the individual plates being maintained by fibrous ceramic cushioning mem- "ice bers. Such assemblies are provided with inorganic spacers on the front portion of the assembly to provide proper design contours. Moreover, during assembly the arc plates, cushioning members and spacers must be maintained under pressure while the composite assembly is bonded by a suitable adhesive. It is evident that such composite arc plate assemblies are relatively costly to manufacture and that the cumulative plate, cushioning member and spacer tolerances may result in rejection of a high proportion of assembled arc plate stacks.

Another problem frequently encountered in the prior art relates to the ultimate use of arc plate assemblies within the arc chute of a circuit breaker. Specifically, the arc chute of a circuit breaker normally includes a pair of arc runners positioned at opposite ends thereof, and the resultant arc chute assembly must fit exactly in the space between such are runners. Because of the relatively poor tolerances which can be achieved when pressuremolding the individual arc plates of an arc plate assembly, engineers are frequently faced with the problem that an arc plate assembly of X plates does not completely fill the space between the arc runners while an arc plate assembly of X +1 arc plates will be greater than the space provided between the pair of arc runners. Thus in many circuit breakers, additional spacing material has to be provided within the arc chute to fill the space remaining between the last arc plate of the arc plate assembly and one of the arc runners.

It is accordingly among the objects of the invention to provide a relatively simple and economical method for molding a ceramic arc plate in a configuration which distributes the molding pressures in a manner such as to minimize the density variations in the resulting are plate and thereby form a product which is relatively free from internal stresses and cracks. I

A further object of the invention is to provide a rela tively simple and economical method for manufacturing an arc chute assembly from such ceramic arc plates, which assembly is resiliently supported along the edges of the adjacent arc plates, in order to minimize fracture of the individual plates by mechanical forces accompanying the circuit interrupter.

Still another object of the instant invention is to provide an arc chute assembly comprised of a plurality of elongated arc plates which define a stack, the thickness of which cannot exceed a predetermined dimension available in the arc chute in which the arc chute assembly is to be used, and wherein the thickness of a predetermined number of the arc plates of this stack may be selectively chosen to match the aggregate thickness of the stack with such predetermined dimension.

Still another object of the instant invention is to provide such an arc plate assembly wherein such predetermined number of arc plates are distributed as evenly as possible at opposite ends of the stack.

An additional object of the invention is to provide an arc plate and an arc plate assembly including a plurality of such plates, produced in accordance with such methods.

Other objects and advantages of the present invention will be apparent from the following detailed description thereof.

In accordance with the invention a ceramic material is pressure-molded into an arc plate having a pair of opposed faces, each of which faces includes a pair of substantially parallel upstanding ribs formed integrally therewith and extending lengthwise of the arc plate, spaced slightly from the marginal edges thereof. Each of the ribs is formed having a face portion disposed subtantially parallel to the adjacent face of the arc plate and inclined wall segments disposed on opposite sides of such face portion, connecting the face portion to the aforesaid are plate face.

Upon molding a ceramic composition, which may be any of those described hereinabove, into the aforesaid configuration the body of the arc plate is formed having higher material densities adjacent the sides of the ribs as compared with the densities of the ribs themselves; it has been found that the resulting density differential, when adjusted .as described below, prevents cracking of the arc plate thus formed.

In order to adjust the aforesaid density differential, the combined thickness of each pair of oppositely disposed ribs formed integrally of the opposed faces of the arc plate is made no greater than about the thickness of the arc plate itself. Each rib occupies a relatively small fraction of the width of the arc plate, the face portion of each rib approximating from about .05% to 2% the width of the entire plate. Moreover, the width of the portions of the arc plate face between each rib and the adjacent marginal edge of the plate is no greater than about the width of the face portion of the adjacent rib and, preferably, is from about Me the width of such face portion to the face portion width itself. An arc plate having limited symmetrical density differentials is thus formed, which plate may be subjected to mechanical stress without cracking in the manner of plates having uncompensated density differentials.

It is additionally preferred to form the inclined wall segments of the upstanding ribs disposed at angles of from about 30 to 60, desirably about 45, with respect to the plane of the adjacent arc plate face, in order that the compressive forces produced during molding are resolved into vectors acting equally into the center of each rib, thus providing substantially uniform minimum density variations in the zones of the arc plate whereat the ribs are formed.

According to a further aspect of the present invention an arc plate assembly is manufactured from the individual arc plates thus produced, by assembling a plurality of such plates with the face portions of the individual ribs adjacent the corresponding marginal edges of the plates aligned with one another, and embedding the marginal edges of such plates in a resilient material which maintains the plates in assembled relation with the aligned face portions of the respective ribs spaced slightly apart from one another such that the aligned planar faces of the arc plates are thereby maintained sufliciently spaced apart so that an arc must follow an elongated tortuous path to pass through the resulting arc plate stack. Because of the slight spacing between adjacent face portions, the resilient material, which is preferably an epoxy resin-containing composition, supports the arc plates in such a manner as to absorb the shock forces accompanying arc formation and thereby prevent the relatively brittle ceramic arc plates from breaking and cracking. Furthermore, the resilient material has thixotropic properties which prevent it from flowing through the small spacing between adjacent face portions during the manufacture of the arc chute assembly. The structure of the individual arc plates thus facilitates ready assemblage of the plurality of plates into a resiliently mounted stack suitable for are extinction.

According to a further aspect of the present invention, the arc plate assembly is constructed bearing in mind the dimensions of the arc chute into which the finished arc chute assembly must eventually be placed. Specifically, the arc chute assembly is comprised of a predetermined combination (dependent upon the particular are chute involved) of normal thickness are plates and extra thick arc plates such that the aggregate thickness of the stack of arc plates exactly matches the dimension of the arc chute involved. Thus no additional filler material has to be provided to take up the space which would otherwise result if the maximum number of normal arc plates which would fit into a given are chute did not take up the entire chute.

ferent embodiments of the present invention to which, however the invention is not limited:

FIGURE 1 is a perspective view of an I-beam or channel-shaped arc plate constructed in accordance with the prior art;

FIGURE 2 is a perspective view of an arc plate having a configuration in accordance with a preferred embodiment of the present invention;

FIGURE 3 is a partial section through an arc plate according to an alternative form of the invention;

FIGURE 4 is a perspective view of an arc plate assembly incorporating the arc plate of FIGURE 2;

FIGURE 5 is an end view showing three of the arc plates of the assembly of FIGURE 4; and

FIGURE 6 is a partially cut away perspective view of an alternative embodiment of the arc plate assembly shown in FIGURE 4.

It is known, as illustrated in FIGURE 1, to provide an arc plate 10 including a pair of oppositely disposed faces 11, each of which has a pair of upstanding ridges 12 disposed along its opposite edges. When such an arc plate is formed by compression molding utilizing steel tooling, for example, the peripheral ridges 12 possess markedly lower densities than the intermediate faces 11. The stresses produced by the unequal density distribution in the ceramic plate thus formed frequently produce fissures or cracks therein.

Such unequal density distribution is avoided, in accordance with this invention, by molding the arc plate into the configuration shown in FIGURES 2, 4, 5 and 6, in which the plate 20 includes a pair of opposed substantially planar faces 21, each of which surfaces includes a pair of substantially parallel upstanding ribs 22 formed integrally therewith. The ribs 22 extend lengthwise of the plate and are spaced slightly from the marginal edges 23 and 24 thereof, each such rib having a face portion 25 disposed substantially parallel to the adjacent face 21, and a pair of inclined wall segments 26 connecting the opposite sides of the face portion 25 to the face 21. As shown most clearly in FIGURES 2 and 4, the arc plates of FIGURE 2 include a cut-out notch 21a, which, as well known in the art, functions to guide and lengthen an are being extinguished.

Sections 27 of each planar face 21, intermediate the marginal edges 23 or 24 and the corresponding ribs 22, as well as the portions of the face 21 disposed inwardly. of such ribs, possess greater densities than the densities of the ribs themslves. The density difiFerentials are, however, adjusted by so molding the arc plate that the combined thickness of each pair of oppositely disposed ribs 22a, 22b is less than the thickness of the plate 20 on which the ribs are formed, and the width of each section 27 is less than the width of the face portion 25 of each rib.

Moreover, the inclined walls 26 of each rib are so formed as to symmetrically taper toward the face portion 25 of the rib, desirably at an angle of about 45 with respect to the plane of the adjacent face 21 of the arc plate.

By forming the inclined wall segments of the rib symmetrically of the face portion thereof, the compressive forces exerted during molding of the plate are resolved equally into the center of the rib, thus minimizing the density variation between the rib and the adjacent face of the arc plate.

In a preferred form of the embodiment illustrated in FIGURE 2, for example, the arc plate 20 was thick and had ribs 22a and 22b of thickness each, formed integrally thereon. The width of portion 25 of each rib was the width of each section 27 was between A and A and the tapered walls 26 of each rib were formed at angles of 45 with respect to the faces of the plate 21.

In accordance with a further embodiment of the invention illustrated in FIGURE 3, the upstanding ribs 32 may be provided in the configuration of a cylindrical segment, having a [face portion 33 constituting an element of the cylindrical segment and extending longitudinally of the arc plate. The segmented wall sections 34 of the rib integrally connect the face portion 33 to the adjacent sections of the face 21 of the plate, preferably abutting the face 21, as indicated above, at angles of 45. Such construction, like that shown in FIGURE 2, distributes the molding force over the surfaces of the arc plate, thereby minimizing the risk of crack formation due to stresses in the molded structure.

A plurality of arc plates having the configuration shown in FIGURE 2 are stacked to produce the arc plate assembly illustrated in FIGURES 4 and 5, with the face portions 25 of the ribs 22 positioned in alignment with one another but spaced slightly apart as at 25a. Sections 27 of the several plates, e.g., plates 20, 40, 60, etc., are embedded in a resilient cement 35, which fills the spaces between the sections 27 of the adjacent arc plates and secures the individual plates in assembled relation. The opposed faces 21 of the adjacent arc plates are thereby sufficiently spaced apart to provide a tortuous path extending through zones 36, 37, etc. between the individual arc plates for extinguishing an are.

As noted above, the resilient cement preferably cornprises an epoxy resin-containing composition which, in addition to supporting the arc plates in assembled relation, is sufficiently flexible to absorb the mechanical shock accompanying arcing without fracture of the individual ceramic plates. The resinous cement acts as a cushioning member between the individual arc plates and allows the assembly to freely expand during circuit interruption and heating. Moreover, as apparent from FIGURE 4 the cement 35 entirely fills the space between the sections 27 of individual arc plates 20, 40, 60, etc. preventing moisture from reaching the completely ceramic interior of the arc plate assembly.

As noted previously, the resilient cement is thixotropic in nature, such that during the manufacturing of the arc chute assembly it will not flow through the small space 25a between adjacent face portions 25 into the interior of the arc plate assemblies.

Referring to FIGURE 6, there is shown an arc plate assembly 42 comprised of a plurality of arc plates 44 each constructed in accordance with the principles discussed. In addition, arc plate assembly 42 includes at opposite ends thereof arc plates 46 and 48 which are substantially thicker in overall dimension than the arc plates 44. As was the case for the arc plate assembly of FIGURE 4, all of the arc plates whether they be the regular arc plates 44 or the thicker arc plates 46, 48 are spaced from one another such as illustrated at 58 to allow the cement 35, rather than the arc plates themselves, to absorb the high mechanical stresses prevalent during interruption- It may be pointed out that in FIGURE 6 the cement 35 is completely embedded about the marginal edges 23, 24 of the arc plates such that a relatively smooth layer 52 of cement is present on each side of the arc plate assembly 42.

The reason for the thicker arc plates 46, 48 will become apparent when one realizes that for a particular arc chute in which the assembly 42 is to be ultimately placed, there is some predetermined dimension identified as L in FIGURE 6 between the arc runners 54 and 56 normally located at each end of the arc chute (not shown). Consequently and because of the relatively poor tolerances which can be achieved in the pressure-molding'of ceramic arc plates, the engineer frequently finds that the given number of regular arc plates, such as 44, does not completely fill the space identified as L in FIGURE 6. Furthermore, and as most frequently happens, the remaining space is not sufficient to accommodate another regular plate such as 44.

In accordance with the instant invention, the engineer, cognizant of the given dimension L, computes the exact number of regular plates 44 and thicker plates 46, 48 which will exactly match the length L.

Thus, and for example, if fifteen regular arc plates left a space of one-quarter inch (MW being less than the thickness of a regular arc plate 44), then the engineer might utilize arc plates 46, 48 each of which was thicker than are plate 44 by one-eighth inch, such that the aggregate thickness of the stack 42 would exactly match and be perfectly received within the space between the runners 54 and 56, designated L. For reasons to be further explained, if an even number of thicker plates such as 46, 48 are required, such plates are evenly distributed at each end of the arc plate assembly 42 (see FIGURE 6 for an example wherein only two such plates are required). If an odd number of thicker plates were required they should be distributed as evenly as possible at the opposite ends of the assembly 42.

Further in accordance with the instant invention, it may be noted that during the interruption process, the greatest amount of ionized gases are liberated in the vicinity immediately adjacent the arc runners 54, 56, that is, immediately adjacent the roots of the arc drawn between the runners. Thus, the greatest pressure buildup of gases occurs at the opposite ends of the are assembly 42. For this reason the thicker arc plates such as 46, 48 are placed at the opposite ends of the assembly 42 to diminish the chance that pressure build-up adjacent the runners could cause an explosion. Thus the thicker arc plates such as 46, 48 which were originally designed to take up an otherwise resulting gap, are simultaneously so positioned so as to increase the safety and reliability of the overall arc plate assembly 42.

In one specific embodiment a number of ceramic arc plates were pressure-molded under a pressure of 10,000 p.s.i., into the configuration shown in FIGURE 2, from v a composition consisting essentially of 35.0% talc, 35.0%

kaolin and 30.0% alumina (an approximate oxide composition of MgO-513%, Al O 37-57%, and SiO 3850%). The compressed arc plate was thereafter fired at 2534 F. After cooling, the plates were assembled in alignment as shown in FIGURES 4 and 5 and cemented together, employing a resilient cement consisting essentially of 41.65% by weight of a liquid epoxy resin (Ciba 502) having an epoxide equivalent of 256 and a viscosity of from 3-6000 cps. at 23 C., 41.65 by weight of a fluid polyamide resin produced by the condensation of poly-carboxylic acids with polyamines (Versamid and 33.4% of an asbestos filler.

It was found that, upon repeating such procedure, no more rejects were produced due to cracking of the relatively brittle arc plates than obtained in the manufacture of flat arc plates, free from depressions and consequent density variations. Moreover, the arc plate assembly thereby formed, upon being subjected to repeated shock forces accompanying arc formation, did not crack or break.

The present invention thus provides a method for molding arc plates and for manufacturing arc plate assemblies incorporating the same, and such arc plates and arc plate assemblies, which minimizes crack formation in the resulting arc plates and reduces the risk of fracture of individual arc plates when subjected to the mechanical forces accompanying the circuit interrupter. The invention further simplifies manufacture of arc plate assemblies, and while eliminating relatively costly inorganic cushioning members between the individual arc plates, provides a compact integral arc plate assembly which may be readily manufactured.

Since certain changes may be made in the embodiments described above without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An arc plate assembly comprising a plurality of elongated arc plates:

each of said plates having a pair of opposed substantially planar faces each of which has a pair of parallel upstanding ribs formed integrally therewith lengthwise of the arc plate and spaced slightly from the marginal edges thereof;

each of said ribs having a face portion disposed substantially parallel to the adjacent planar face of the arc plate, and, along the opposite edges of said face portion, inclined wall segments which taper outwardly from and connect the face portion to the adjacent planar face of the arc plate;

the planar faces of said are plates being assembled parallel to one another with the face portions of said ribs which are disposed adjacent corresponding marginal edges of the arc plates being aligned with one another, the marginal edges of said plates being embedded in a resilient cement which maintains the plates in assembled relation with the aligned face portions of the respective ribs spaced slightly apart from one another.

2. The are plate assembly of claim 1, wherein said inclined wall segments are disposed at angles of from 30 to 60 with respect to the planar faces of said are plates.

3. The are plate assembly of claim 1, wherein the thickness of each of said ribs is no greater than one-half the thickness of the respective arc plate.

4. The arc plate assembly of claim 3, wherein the width of the portion of each arc plate planar face 'between the marginal edge thereof and the adjacent rib is no greater than the width of the face pontion of the adjacent rib.

5. The are plate assembly of olaim 1, wherein said plurality of arc plates maintained in said assembled relation by said resilient cement defines a stack, the aggregate thickness of which must be no greater than a predetermined dimension; and wherein the thickness of a predetermined number of arc plates in said stack is selectively chosen to match the aggregate thickness of said stack with said predetermined dimension.

6. The are plate assembly of claim 5, wherein said predetermined number of arc plates are distributed as evenly as possible at opposite ends of said stack.

References Cited by the Examiner UNITED STATES PATENTS 2,744,983 5/1956 Taylor 200-144 2,959,653 11/ 1960 Stringfellow 200-1 t4 FOREIGN PATENTS 1,160,990 3/1958 France.

ROBERT K. SCHAEFER, Primary Examiner. 

1. AN ARC PLATE ASSEMBLY COMPRISING A PLURALITY OF ELONGATED ARC PLATES: EACH OF SAID PLATES HAVING A PAIR OF OPPOSED SUBSTANTIALLY PLANAR FACES EACH OF WHICH HAS A PAIR OF PARALLEL UPSTANDING RIBS FORMED INTEGRALLY THEREWITH LENGTHWISE OF THE ARC PLATE AND SPACED SLIGHTLY FROM THE MARGINAL EDGES THEREOF; EACH OF SAID RIBS HAVING A FACE PORTION DISPOSED SUBSTANTIALLY PARALLEL TO THE ADJACENT PLANAR FACE OF THE ARC PLATE, AND, ALONG THE OPPOSITE EDGES OF SAID FACE PORTION, INCLINED WALL SEGMENTS WHICH TAPER OUTWARDLY FROM AND CONNECT THE FACE PORTION TO THE ADJCENT PLANAR FACE OF THE ARC PLATE; THE PLANAR FACES OF SAID ARC PLATES BEING ASSEMBLED PARALLEL TO ONE ANOTHER WITH THE FACE PORTIONS OF SAID RIBS WHICH ARE DISPOSED ADJACENT CORRESPONDING MARGINAL EDGES OF THE ARC PLATES BEING ALIGNED WITH ONE ANOTHER, THE MARGINAL EDGES OF SAID PLATES BEING EMBEDDED IN A RESILIENT CEMENT WHICH MAINTAINS THE PLATES IN ASSEMBLED RELATION WITH THE ALIGNED FACE PORTIONS OF THE RESPECTIVE RIBS SPACED SLIGHTLY APART FROM ONE ANOTHER. 